As the title of this post says, this flower in the photograph above is a red one. Since I wanted a black and white photograph, which favours contrast and clearly distinguishable black or white objects, I used a red filter in front of the lens in order to make the red petals look white. The green foliage, in contrast, becomes darker.
This works by basically turning the whole image into a red channel. White and red pass through and print their luminosity on the sensor, while other colours are absorbed by the filter, hence becoming darker. In fact, you are shooting the red channel only, and this requires compensation for the lost light, so you must compensate.
Here is a colour version of the photograph above. This is not the version which I converted.
If I had simply converted the colour image into black and white without using the filter, the red flower and the green foliage would have been a very similar shade of grey, and I would have to separate them by tweaking in software. Not nice.
Simply converting a colour image to black and white, say in Adobe Lightroom, allows you to tone each colour separately in grayscale. Using a filter like this makes the entire image red, so you can only really tone the whole thing up or down using that technique. You will still have some slight control over tones and contrast. The next time you are making a black and white photograph, experiment with both techniques on the same image to decide which gives you your own desired effect.
Here is the filter in question. You can use a regular circular red filter with threads to fit it directly on the lens, or you can use something like the Cokin filters. This one is similar to Cokin in that it is acrylic. The filter itself is only slightly smaller than my lens's filter threads, so I used the plastic holder to handhold it in front of the lens while shooting. The filter and its plastic holder were borrowed from my Durst M370 colour enlarger.
Here is the same image, retaken with the red filter in front of the lens. The filter is so good that it will only really let red go through. Even white becomes red, and this is why red and white become equals, so when you convert to black and white, they look the same. This makes the red flower look much lighter, and makes foliage a bit darker, just the way I want it.
Again, here is the red photograph simply converted to black and white. Some tweaks were made to tones, I believe.
Canon EOS M mirrorless camera
Tamron 90mm f2.8 1:1 Macro lens
YongNuo 622c remote e-TTL triggers
Canon 430EX II x1 in manual mode
Enlarger's red filter
Adobe Lightroom 5